1. Field of the Invention
This invention pertains generally to the field of implantable body tissue stimulators, e.g., cardiac single chamber and dual chamber pacemakers and cardioverters, and more specifically to a method and apparatus for monitoring associated implanted lead and electrode impedance characteristics for analysis and modification of lead selection, operating mode and output pulse parameters.
2. Description of the Prior Art
Implantable body tissue stimulators usually include an electrode in contact with tissue to be stimulated mounted on a lead or housing containing an electrical conductor to be connected between the electrode and a source of stimulating energy. Usually the stimulating energy source is a fully implanted pulse generator which provides a capacitive discharge output pulse or shock to the electrode through the lead. Such pulse generators and stimulators include cardiac pacemakers, nerve and muscle stimulators and cardioverters, including combined pacemaker-cardioverter-defibrillator devices. Moreover, such devices usually include sense amplifiers coupled to the leads and electrodes specifically designed to sense spontaneous electrical activity of skeletal muscle and heart tissue. The integrity of the lead and electrode as measured by its impedance can critically affect both stimulation and sensing performance.
The technology of cardiac pacemakers implantable nerve stimulators and pacemakers-cardioverters-defibrillators has developed a high level of sophistication of system performance. The current generation of cardiac pacemakers and pacemaker-cardioverter-defibrillators incorporates microprocessors and related circuitry to sense and stimulate heart activity under a variety cf physiological conditions. These devices may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A background description of modern cardiac pacemaker technology is set forth in U.S. Pat. No. 4,958,632, which patent is incorporated herein by reference. It is a primary goal of programmable, multiple-mode, demand-type, cardiac pacemakers to accomodate the changing requirements of a diseased or malfunctioning heart. For example, single chamber, fixed rate pacers have been used extensively in the past to correct bradycardia, or slow heart rates Demand pacing is employed to avoid competing rhythms in patients who have some cardiac activity. Dual chamber pacing is used to treat complete or intermittent heart block by maintaining atrio-ventricular (AV) synchrony. Various other parameters (such as rate, pulse amplitude or width, sensitivity, refractory, etc.) may also need to be altered from time to time to custom-fit the pacemaker to each patient.
Programmability has also been incorporated into pacemakers to select the type of electrodes implanted, either unipolar or bipolar. A unipolar lead is one in which stimulation occurs between the cathode tip electrode and the pacemaker case, or anode. A bipolar lead is one in which stimulation occurs between the cathode tip electrode and an anode, ring electrode spaced approximately one inch from the tip electrode. Physicians select one lead type over the other for a variety of reasons. A unipolar lead may be chosen due to its advantage of being physically smaller and more flexible and, therefore, easier to implant. Unipolar leads also have the advantage of being less vector sensitive for intrinsic complexes (particularly premature ventricular and atrial contractions) due to the larger dipole. On the other hand, bipolar leads provide superior noise immunity to myopotentials and electromagnetic interference. It is also known that bipolar leads eliminate pectoral muscle stimulation, however, there has also been an occasional report of diaphragmatic stimulation. Since these leads are inaccessible after implantation (except by surgical procedure), bipolar leads may advantageously be used in either unipolar or bipolar configurations through noninvasive reprogramming of the implanted pulse generator.
While electronic circuitry can be, and is, incorporated within the pacemaker itself for exercising or testing various circuit components (such as the status of battery power sources, and the effectiveness of various amplifiers, waveform shaping stages and the like), it is often more difficult to test the integrity of the leads and implanted electrodes to which the pacemaker is coupled in order to verify that such leads and electrodes can function to allow for the desired pacing operation.
At the implantation of electrode systems, minor damage is sometimes incurred which may affect the lead s electrical insulation. This type of damage may go undetected and be without present effect on the implanted system, but the condition may manifest itself after extended time in service. When a breakdown or significant degradation of the pacemaker lead insulation occurs, it can result in a loss of sensing of intrinsic cardiac events or a loss of capture due to a lessened amount of energy reaching the cardiac tissue. Based on the underlying rhythm of the patient, this may have serious or even disastrous results. The reduced output energy reaching the heart is due to partial energy being shunted to other areas through the insulation opening.
Other types of damage can also occur to a pacemaker lead at implantation or later. A fracture in a conductor coil can affect operation by reducing the energy output to the cardiac tissue by causing a substantial increase in the lead resistance to current flow. A partial fracture will cause a reduction in output energy, while a complete fracture will result in no energy reaching the heart due to an infinite resistance (open circuit) Another type of detectable error relates to the failure of the electrode tip to be in proper contact with the heart wall.
It has been routine to measure lead impedance or stimulation threshold at the time of implantation to permit optimizing the location of the pacing lead and to maximize longevity of the pacer. These acute measurements are made with an oscilloscope or, more frequently, with a special instrument called a pacing system analyzer or PSA.
Later, after the lead has "healed" into the heart tissue, the margin of capture and delivered energy are estimated by the physician. With the advent of output pulse programmable pacers, physicians have been able to adapt the pacer's output to the threshold requirements of the patient and thus prolong the longevity of the implanted device.
To assist the physician with followup care, modern pacers use internal circuitry to monitor the output pulse parameters and to telemeter this information to the physician via a programmer. This information is used in assesing the performance of the pacemaker and the associated lead.
A problem which is presented by this technology is a discrepancy between the measured values of lead current and delivered energy presented by the programmer, PSA and oscilloscope. These differences result primarily from different measurement methodologies.
Typically, prior art pacers place a series resistor in the output path of the lead current, as disclosed, for example, in U.S. Pat. No. 4,140,131. During the delivery of a pacing stimulus, the voltage drop across the resistor is measured and telemetered out. This technique requires the use of a high precision resistor to reduce measurement errors. It also introduces a component whose failure can lead to an undesirable "no output" condition. Also, such a system is wasteful of output energy because of the inclusion of the measuring resistor.
Particular methods and apparatus for scanning the implanted leads of a pacemaker system to determine lead impedance and to detect abnormalities which may signal degradation and impending failure of pacemaker leads are the subject of the above-referenced '632 patent and U.S. Pat. Nos. 4,949,720; 4,899,750, and 5,003,975, all incorporated herein by reference.
Briefly, the '750 patent discloses systems for measuring the output voltage drop delivered to the pacing lead during pacing and determining the lead impedance from that measurement. The thus-determined lead impedance is compared with a moving average of the measured parameter and any deviation from that average by more than a predetermined amount is considered an anomaly. Three such anomalies in succession result in an event being counted in a first event counter for future consideration by a doctor during a patient checkup or the like. The system also monitors sensed heart signals and counts as a notable event any deviations in slope of the heart signal by more than a predetermined amount. These latter events are counted in a second counter to provide information for future reference. Thus, the '750 patent system determines the integrity of the implanted leads and electrodes by making measurements during both the pacing and sensing time intervals of the pacemaker timing cycle.
The '632 patent discloses measuring lead impedance as a function of the time that it takes to recharge the discharged output capacitor to full voltage, and automatically switching pacing electrodes or modes of operation if the time is excessively long (excessively high impedance) or excessively short (exclusively low impedance) or upon loss of capture. The '975 patent discloses similar responses to excessive or insufficient lead impedances, using the technique of the '750 patent.
The '720 patent discloses a lead impedance measuring circuit including a large number (typically 200) of FET transistors operated in parallel to discharge a capacitor through the heart tissue. Pacer lead current is monitored by measuring the current through a small number (typically 2) of these transistors. The current monitoring function is performed by a current-to-voltage converter coupled to an analog-to-digital converter which may make one or more voltage measurements during the output pulse. The ability to time the measurement with respect to the leading and trailing eges of the output pulse provides flexibility to match the telemetered pacing current data with operating room measurements thus reducing the confusion that discrepancies can cause. Additionally, the voltage developed by the current-to-voltage converter may be applied through additional circuitry to the gates of the FETS to provide for a constant current output pulse.
The aforementioned '750, '975 and '720 patents employed a technique for the calculation of lead and lead electrode-tissue interface impedance through measurement of the initial and final voltages across a coupling capacitor when the lead system is paced with a fixed pulse width pacing pulse. The voltage measurement is then translated into a current based on the capacitance value on the pulse width. The lead impedance is then calculated as the average voltage divided by the average current, from which known switch impedances are subtracted. The methods illustrated by these patents have several error items which contribute to overall inaccuracy of as much as .+-.20% in the 100-1000 ohm load range. These error terms include the impedance approximation equation (which avoids the natural log function in order to save on software), capacitance tolerance, analog to digital conversion accuracy, sampled voltage gain stage accuracy, sample and hold leakdown, voltage reference drift and pulse width tolerances of the programmed pacing pulse and the pulse generator circuit timeout thereof.
In commonly assigned co-pending U.S. patent application Ser. No. 619,494 filed Nov. 29, 1990, in the names of Wayne et al it is proposed to calculate lead impedance from lead current computed by measuring output capacitor discharge time over a continued output capacitor discharge occurring from the trailing edge of the pacing pulse until a threshold (which is automatically set to reflect the voltage drop occurring during the preceding output pulse) is reached.
Measurement of the lead impedance (including the impedance contributions of the electrical feedthrough and connector block components, the electrical connection between the connector block components and the lead conductor terminal pin physically attached thereto at implant, the lead conductor itself and its interconnections with the connector pin and electrode and the electrode-tissue interface which, as described above, all may change over time for a number of reasons) in an implanted pacing system allows the user to determine changes in characteristics in the lead as well as the electrode-tissue interface trends over a period of chronic implant. In implantable pacemaker pulse generators, measurement of parameters which are used to calculate the impedance of the lead/heart system must be accomplished in an accurate and precise manner without the aid of external instruments.
Impedance measurement of lead and electrode systems in other tissue stimulation contexts, particularly in high energy cardioversion and defibrillation, has been addressed in only a limited fashion. In external cardioversion and defibrillation, it is known to measure impedance to make certain that good electrode contact with the patient s body exists before applying the shock Implantable cardioversion and defibrillation electrodes are also subject to fracture or shorting out, and it would be desirable to be able to periodically measure lead impedance in order to detect changes reflecting impending or actual failures.